sha1_name.c: add support for disambiguating other types

sha1_name.c: add support for disambiguating other types

This teaches the revision parser that in "$name:$path" (used for a
blob object name), "$name" must be a tree-ish.

There are many more places where we know what types of objects are
called for.  This patch adds support for "commit", "treeish", "tree",
and "blob", which could be used in the following contexts:

 - "git apply --build-fake-ancestor" reads the "index" lines from
   the patch; they must name blob objects (not even "blob-ish");

 - "git commit-tree" reads a tree object name (not "tree-ish"), and
   zero or more commit object names (not "committish");

 - "git reset $rev" wants a committish; "git reset $rev -- $path"
   wants a treeish.

They will come in later patches in the series.

Signed-off-by: Junio C Hamano <gitster@pobox.com>